Oil-free turbomachinery requires high-speed operations to achieve meaningful power and efficiency. Bearings may be used in a rotating assembly of turbomachinery between the rotating and stationary components. It is conventionally known to use a foil bearing as a bearing for a rotating member that rotates at a high speed (greater than 10,000 rpm). The foil bearing comprises a bearing sleeve lined with a plurality of foils including a top foil for forming a bearing surface and supporting a journal of the rotating member (a shaft) by means of pressure of a fluid drawn in between the journal and the top foil as the rotating member rotates. The foil bearing and the journal form a foil journal bearing assembly. The journal and top foil are in contact when stationary and at low journal rotational speeds. When the journal rotates at an optimum operational speed, the top foil and journal separate from each other to form an air gap (also referred to herein as “an operational running clearance.”) As the air gap between the top foil and the journal grows, the pressurized fluid is drawn in to serve as a load support and act as a lubricant (instead of oil) to the rotating member and surrounding static components. The journal rotates within the bearing sleeve providing, in the interest of aerodynamic efficiency, only a desirably small operational running clearance between the rotating journal and top foil.
Unfortunately, when a conventional journal having a constant uniform diameter rotates at high speed, centrifugal forces may result in journal deflection. High-speed operations can also cause a significant temperature increase, due to large viscous heat dissipation in the fluid film between the journal and the foil bearing, as well as centrifugal growth of the journal (and decrease of film thickness). High temperature growth of the journal may result in sections of the journal growing in length axially, as well as radially outwardly producing, for example, a “distorted journal” such as, for example, a saddle-shaped journal or a concave journal. As used herein, “high temperatures” refers to temperatures above 250° F. Such elastic deformation or deflection of the journal is sometimes referred to as “flowering”. As used herein, the term “deflection” and the like includes distortion, elastic deformation, flowering, or a combination thereof, unless otherwise specified. Journal deflection may result in the operational running clearance being made non-uniform along the length of the journal and/or around the circumference of the journal, i.e., insufficient in some areas and excessive in others, thereby upsetting the proper functioning of the foil journal bearing assembly, and possibly resulting in damage and even failure of the rotating assembly and/or foil journal bearing assembly. The journal may also be misaligned, also resulting in journal end-loading. Journal deflection and/or misalignment results in the distorted journal. The distorted journal may rub against the foil bearing that was designed for the conventional journal, causing uneven journal wear, particularly at the ends thereof (i.e., “end-loading”). The distorted journal may impose an uneven load distribution on the bearing surface, increasing the heat generated and thus accelerating bearing wear. Dynamic properties of the foil bearing, such as stiffness, damping, and load capacity may also be detrimentally affected by the distorted journal. Thus, journal deflection, caused primarily by high-speed centrifugal forces and thermal expansion, and journal misalignment, have a large influence on the performance of rotating and foil journal bearing assemblies in turbomachinery.
Accordingly, it is desirable to provide rotating assemblies of high-speed turbomachinery, foil journal bearing assemblies thereof, and methods for producing specially configured journals of the foil journal bearing assemblies. It is also desirable that the journals be configured to resist operational deflection that may occur during high speed and high temperature operation thereby maintaining a substantially constant uniform diameter and uniform operational running clearance and/or to reduce the effects of journal misalignment, thereby improving performance of the rotating and foil journal bearing assemblies in turbomachinery.